Low frequency amplifier



April 10, 1945. H. A. 512005 I 7 mw FREQUENCY AMPLIFIER Filed Au 3, 1937 2 sheets-Sheet 1 INVENTOR HENRICUS A BROOS ATTORNEY- 10, 1945. H, BRQOS I 2,373,534

' LOW FREQUENCY AMPLIFIER Filed Aug. 3, 1937 2 SheecsQSheet 2 INVEMTOR HENRMIUS A. 8. 2005 ATTORNEY salve.

Patented Apr. 1%, 1945 UNITED STATES PATENT omen:

I Low/v raaozzlv o ramrnmua Q lands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application August a, 1937, Serial No. 157,078 In Germany August 3, 1936 It is known to give the grid of an amplifying tube the required negative bias by interposing a resistance in the supply .lead for the cathode,

which resistance is traversed by the anode current, and by connecting the grid to the resistance end on the anode side. This resistance causes a negative back-coupling for the oscillations to be amplified. For this reason it was customary to connect a large condenser n parallel with the resistance. It has been found, however, that the decoupling in the region of the lower frequencies of the low frequency range is all but complete, so

that the amplification is never uniform.

According to the invention this drawback is obviated by coupling the resistance to the grid circuit of a preceding valve so that unevenn in the amplification caused by the combination of the'resistance and of the decoupling condenser are at least partly compensated since the positive feedback by way of the preceding valve increases with decreasing frequency in ex,-. actiy the same way as does the negative feedback between the output and input circuits of the final invention will be more clearly understood by .erence to the accompanying drawings representing, by way of example, several forms of construction thereof. In the drawings, Fig. 1

shows an amplifier employing separate controls for amplifier input and positive feedback respectively; Fig. 2 a modification of the circuit of Fig. 1 in which negative feedback is employed; Fig. 3 is a further modification wherein a single control is used for amplifier input, negative feedback and positive feedback; Fig. 4 shows a modification with common control for amplifier input and negative feedback and a separate control over the low frequency feedback; Fig. 5 is a further modification of ,the circuit shown in Fig. 4;

and Fig. 6 is a modification of the circuit of Fig. 3 applied to a push-pull output stage.

'In Fig. 1, the penultimate tube of a low frequency amplifier is denoted by I and the terminal valve by 2. The anode circuit of the valve 2 comprises the primary winding'of a transformer 3 whose secondary winding is connected to a loudspeaker. The arrangement is adapted both for receiving purposes and for record reproduction. Changing-over is effected by means of the switch 4. I2 is a tuned circuit which is preferably coupled to the final stage of the intermediate frequency amplifier of a receiving set. The voltages occurring thereover are rectified by a detector diode incorporated in the valve I. A pick-up is denoted by II. The rectified signals are supplied 4 Claims. (01. 179-171) ,This resistance is connected to the grid of the valve I through a condenser by means of an adjusting contact. The anode circuit of the valve I is coupled in the usual way to the grid. circuit of the tube 2. To produce a negative bias for the grid of the valve 2 resistances 6 and I are provided to which a'condenser 8 is connected in parallel, and which are traversed by the anode current of the valve 2. The cathode of the valve I 18 connected to an adjustable point of the resistance I,- so that a positive back-coupling is obtained between the valves I and 2, which is most effective for low frequencies. The operation can be controlled by adjusting the contact on the resistance 1.

Pig. 2 shows a circuit arrangement which substantially corresponds to that represented in Fig. l, but in whichthe electrode of the condenser 8 gal I fective for the lower frequen 2 connected to an adjustable point III of the resistance 9 forming part of the grid circuit of the valve I. In this manner a negative backcoupling is obtained which is particularly effective for high frequencies, and is practically zero for low frequencies. Thus the amplifying characteristic can still be improved. Similarly to Fig. 1 the sound strength can be controlled by shifting the contact on the resistance 5, whereas the degree of back-coupling can be controlled by shifting the contact III on the resistance 9.

Fig. 3 represents a circuit arrangement in which the quality is controlled automatically in accordance with the'sound strength. In this figure the signals to be amplified are supplied to a resistance I3 one of whose ends is earthed, whereas the other end is connected to the upper end of the resistance 5. When the contact on the resistance 5 occupies the bottom'.,position the complete back-coupling is operativ'in the input circuit of the valve I.. In this ic'ase the sound strength is minimum. Theratiolbetween the strength of the low sounds, and. thatof the high sounds is maximum which is jus desired. when the said contact is in its upperjposition the negative back-coupling is a whereas the positive back-coupling which? particularly effullyopnsured between blification is erative, so that's. suitable rati low and high sounds fan maximum also fori'lwe k Yijwhich the contact at the grape endgof e re'sistance 5, since the negative back cbupliiigissmall.

Figure 4 represents anarrangement according to the invention, in which the final stage comprises two push-pull connected valves H and 15. To avoid distortion the output voltage of the two terminal valves should be coupled back, which is effected by connecting the cathodes across resistances l8 and "Is to the resistances I8 and ll. These last-mentioned resistances form part of the input circuit of .the valve I. The common point of these resistances is earthed. A simple consideration shows that in this manner the two voltages are coupled back in the suitable-phase if the polarity of transformer 20 is correctly chosen, as must be determined by the method of trial and error; The cathodes are connected through a condenser 2| due to which they have substantially the same potential for higher frequencies. The ratio between the higher and the lower frequencies in the output circuit can be controlled by means'of a resistance -22 connected in parallel with the resistances i and i! or in parallel with C21.

Fig. 5 shows a circuit arrangement which comprises, moreover, a negative back-coupling for the high frequencies. This is effected through the condensers 23 and 24 connected between the cathode of the valve and the cathode of the valve l, and between the cathode of the valve l4 and the bdttom end of the resistance i! respectively. The back-coupling can be controlled bymeans of a variable resistance 25-which is interposed between the two cathodes, but may also be connected parallel with the resistances it and H. In the arrangements represented in Figs; 4 and 5 is illustrated, moreover, how'tiie back-coupling can be rendered dependent on the sliding contact on the resistance 5. The feedback of resistance 5. In Fig. 5 exactly the same control of feedback results from the movement of the slider to resistance 5, since the source of feedback voltage is connected on the one hand directly to the cathode of the'ilrst' tube and on the. other hand through the same arrangement,

of resistance as described before to the grid.

Fig. 6 represents a circuit arrangement comprising a. negative back-coupling only for the higher frequencies in the grid circuit of the valve I across'the resistances l6 and I1. back-coupling for the low frequencies is not provided in this arrangement.

-What is claimed is:

1. In combination, an electron discharge de- A positive vice having input electrodes, of. potentiometer,

means to supply'slgnal oscillations across said potentiometer, said input electrodes being variably tapped from said potentiometer, thereby to vary the ,portion 01' said signal oscillations on said potentiometer supplied to said input electrodes, an output circuit, means to supply voltage from said output circuit to said input electrodes to produce degeneration, and'means toreduce said voltage supplied to said input electrodes substam tially to zero in one position 01 said potentiometer, thereby substantially to remove said degeneration.

2. In an audio amplifying system, an audio amplifier having an audio input circuit and an audio output circuit, said input circuit including an impedance across which is developed audio signal voltage, a resistive path in shunt with said impedance, 9. degenerative signal voltage feedback path connected between said output circuit and an intermediate point of said rean adjustable connection between the input electrode of said amplifier and a portion of said path between one endthereof and said point, said amplifier including at least two cascad tubes each having a cathode] input electr and anode, and resistive means connecting the oathodes of both tubes in common to the opposite end of said shunt resistive path.

i. In c-xi-mbination, an electron discharge vice having input electrodes, at potentiometer,

to supply signal'oscillations across'said potentiometer, said input electrodes being variably tapped from said potentiometer, thereby to vary the portion of said signal oscillations said potentiometer supplied to said input elec trodes, an output circuit, means to supply voltage from said output circuit to said input electrodes to produce degeneration, and means to reduce said voltagesuppliedtosaid input electrodes substantiallyto zero in one position of said potentiometer 'therebyisubstantially toremove said degeneration, said output circuit including a second electroni discharge device, a bias resistor common to'fthe space current paths .of both devices, andsaid degeneration means consisting of a condenser'connect ed eflectively v in shut relation to a path consisting of 'said bias resistor in series with ayportion of the ,potem tiometer. HENRICUS ADKINS BROOS. 

